1. Field of the Invention
The present invention relates to a method for continuous casting of steel wherein molten steel is solidified by cooling means and produced solidified shells are successively withdrawn and an apparatus therefor, and more particularly to a method for continuous casting of steel by the use of cooling means such as a cooled roll, a cooled mold and the like and an apparatus therefor.
2. Description of the Related Art
Various methods for continuous casting of steel using a cooled roll and a cooled mold have been reported. Japanese Patent Application Laid Open No. 284469/89 discloses a method for continuous casting of steel wherein a refractory nozzle and a cooled mold connected to said refractory nozzle are used. In this method, the refractory nozzle is means for feeding molten steel and the mold is cooling means. Japanese Patent Application Laid Open No. 210154/89 discloses a method for continuously manufacturing a steel sheet by solidifying molten steel on a circumferential surface of a rotating cooled roll. In this method, a refractory dam is placed near an end face of said cooled roll to hold molten steel on the surface of said roll, and the refractory dam is means for feeding molten steel, and the cooled roll is means for cooling molten steel.
Problems are generally raised that a solidified shell is generated at a zone where cooling means such as a cooled mold and a cooled roll, means for feeding molten steel such as a refractory nozzle and a refractory dam and molten steel contact each other, which gives rise to a great deterioration of surface properties of a cast product.
It is an object of both the prior art methods in the Japanese Patent Application Laid Open No. 284469/89 and the Japanese Patent Application Laid Open No. 210154/89 not to cause defects in a cast porduct. The prior art methods are methods wherein a solidified shell is not generated at a zone where means for cooling molten steel and means for feeding molten steel and molten steel contact each other.
The method for continuous casting of steel disclosed in the Japanese Patent Application Laid Open No. 284469/89 will now be described with specific reference to FIG. 4. FIG. 4 is a partially sectional view illustrating a zone adjacent to a connecting portion where a refractory nozzle for feeding molten steel to a cooled mold is connected to the cooled mold. A coil 4 is placed near the connecting portion where the refractory nozzle 1 is connected to the cooled mold 2. That is, the coil 4 is positioned inside the refractory nozzle 1 just in front of an inlet port of the cooled mold 2. A high-frequency electric current is flowed through the coil 4, thereby generating a magnetic field. A magnetic pressure is generated on the part of molten steel near by a zone where the refractory nozzle 1, cooled mold 2 and molten steel contact each other by interaction between said magnetic field and said molten steel. Molten steel 5 at said zone is pressed toward inside, whereby a space 7 is formed. A solidified shell 6 is hard to be generated at the zone where the refractory nozzle 1 and the cooled mold 5 and molten steel contact each other due to formation of the space 7. The molten steel 5 begins to be solidified from a position adjacent to the space 7 on the inner surface of the mold 2. There is no surface defect such as a draw mark referred to as a cold shut in a withdrawn billet and the billet with good surface properties can be obtained.
The method disclosed in the Japanese Patent Application Laid Open No. 210154 will now be described with specific reference to FIG. 5. FIG. 5 is a partially sectional view illustrating a zone adjacent to a cooled roll and a refractory dam placed near an end face of said cooled roll. The cooled roll 20 is immersed into molten steel 5. The refractory dam 21 is positioned along both the end faces of said cooled rolls 20 so that the molten steel 5 cannot penetrate between the dam 21 and the side of said roll 20. A coil 4 is positioned outside the refractory dam 21. A high-frequency electric current is flowed through the coil 4 whereby a magnetic pressure is generated. The molten steel 5 at a zone adjacent to the end face of the cooled roll 20 and adjacent to the refractory dam 21 is pressed to the inside, thereby a space 7 is formed. A solidified shell is hard to be generated at the zone adjacent to the end face of the cooled roll 20 and adjacent to the refractory dam 21 due to formation of the space 7, which solves a problem of deterioration of surface properties of a steel sheet. That is, there cannot be a problem that the solidified shells generated at the zone adjacent to the cooled roll 20 and adjacent to the refractory dam 21 stick to each other and is connected to each other, that a connecting portion of the solidified shells is broken by rotation of the cooled roll 20, and that end faces of the steel sheet in the direction of the breadth of the steel sheet are made zigzag by repeated sticking and breaking of the solidified shells.
However, since a magnetic field is only generated by simply flowing an electric current through the coil 4, the magnetic field generated is scattered. Therefore, an effective magnetic pressure cannot be caused to act on the zone where cooling means such as the cooled roll 20 and cooled mold 2 and feeding means such as the refractory nozzle 1, refractory dam 21 and the molten steel 5 contact each other. In order to generate a magnetic pressure strong enough to be able to form a space 7 where there is no molten steel at the zone where the cooling means, feeding means and the molten steel 5 contact each other, a great high-frequency electric current shoud be flowed through the coil 4, which requires a high-frequency power source of large capacity.